In the operation of screw machines of known design, the bar stock is fed into the machine in long lengths and has had a very considerable tendency to vibrate transverse to its length. Usually, the rotating bar stock is passed through an elongated guide tube held stationary at the inlet side of the screw machine. The rotating bar stock has a loose fit inside the non-rotating guide tube and it is free to vibrate transversely, striking the tube and thereby causing appreciable noise and often damaging the bar stock itself, particularly if it is of hexagonal or other polygonal, sharp cornered cross-section. Excessive noise in a machine shop is an occupational hazard which can be damaging to the safety, health and well-being of workers there and is contrary to federal policy, as expressed in the Occupational Safety and Health Act.